The south-west constituency is a marginal seat for all three main parties. Labour holds none of the three boroughs of which it is made up and the assembly battle is between the incumbent Tory, Tony Arbour, and his Lib Dem challenger, Stephen Knight.

Yet Labour holds two of the five parliamentary seats it comprises and the south-west favoured Ken Livingstone for mayor in 2004. It is key territory for him if he's to keep Boris Johnson at bay.

He needs support from Lib Dem voters: the type who've made Vince Cable the MP for Twickenham and raised the orange flag at Richmond council. It was no accident that he launched his environment manifesto within the latter's boundaries, praising the administration that took power two years ago for making gas-guzzlers pay higher parking fees.

Its deputy leader, Stephen Knight, is Arbour's challenger this time and he needs Labour tactical voters to get in. Livingstone needs Lib Dem tactical voters. Both want to woo persuadable Tories too. It's a battleground as complex as the seat itself.

Arbour is a local man and lifelong local politician. He's been a Richmond councillor since 1968 when he was 22, represented Surbiton on the Greater London Council – from 1983 until 1986 when Margaret Thatcher abolished it to get rid of Livingstone – and was a lecturer at local Kingston university business school until securing his assembly seat in 2000.

A member of the metropolitan police authority, which Johnson has promised to chair, he conforms gladly to the controversial Johnson line that there's too much "political correctness" at the Met and that:"Our police should be nicking, not box ticking."

Knight has been busy attacking on two fronts. He has criticised Arbour for opposing the proposed south-west branch of the planned crossrail line when he was Richmond council leader prior to the Tories' defeat, making the case the local businesses will be expected to help pay for the project anyway.

More recently, he has pitched in to Gordon Brown over the 10p tax rate debacle. "Thousands of residents on low incomes in Hounslow are now receiving sharply increased tax demands and many are facing big increases in bills for gas, electricity and transport," he said.

The specific mention of Hounslow is significant. Hounslow council is the one where Labour remains strongest and would have hung on in 2006 had the Tories not done a deal with the local Independent Community Group.

The third of the south-west's three boroughs is a royal one – Kingston upon Thames, which the Lib Dems held on to despite a Tory surge in 2006. As well as the town of Kingston itself, which David Cameron visited with Johnson during the campaign, it includes Henry VIII's riverside palace Hampton Court and the spur of Surrey land containing Hook and Chessington, home of the world of adventures theme park and zoo.

Will such places resist Boris's flattering overtures? Has he won over Feltham to the west, Teddington at its centre, sylvan Kew or Chiswick on its north-east border, the nearest point in this green, often genteel and very suburban seat to the inner-city?

Or, faced with the choice between Livingstone, the old devil they know, and Johnson, the younger unknown quantity, will the south-west's centrist swing voters help keep Livingstone in power, perhaps giving him the second preference votes he craves? We'll soon know.